The present application relates to an image display device employed for allowing an observer to observe a two-dimensional image formed by an image forming device or the like.
A virtual image display device (image display device) employing a hologram diffraction grating for allowing an observer to observe a two-dimensional image formed by an image forming device by a virtual image optical system as an enlarged virtual image has been disclosed with Japanese Unexamined Patent Application Publication No. 2007-94175.
Such an image display device 1010 basically includes, as shown in FIG. 25A, an image forming device 1011 for displaying an image, collimating optical system 1012, and virtual image optical system (optical device 20) which light displayed on the image forming device 1011 is input to, and guided to a pupil 50 of an observer. The optical device 20 includes a light guide plate 21, and a first diffraction grating member 30 and second diffraction grating member 40, which are made up of a reflection-type volume hologram diffraction grating provided on the light guide plate 21. Light emitted from each pixel of the image forming device 1011 is input to the collimating optical system 1012, multiple parallel beams of which the angles input to the light guide plate 21 differ are generated by the collimating optical system 1012, and are input to the light guide plate 21. The parallel beams are input to one optical face (first face) 22 of the light guide plate 21, and are output therefrom. On the other hand, the first diffraction grating member 30 and second diffraction grating member 40 are attached to the other optical face (second face) 23 of the light guide plate 21 which are parallel to the first face 22 of the light guide plate 21.
Now, when assuming that the center of the first diffraction grating member 30 is the origin Oi, a normal of the first diffraction grating member 30 passing through the origin Oi with the direction toward the collimating optical system side as the positive direction is an Xi axis, and the axis line of the light guide plate with the direction toward the second diffraction grating member side as the positive direction is a Yi axis, the optical axis of the collimating optical system 1012 is identical to the Xi axis, and moreover, the optical axis of the collimating optical system 1012 passes through the center of the image forming device 1011.
The multiple parallel beams of which the angles input to the light guide plate 21 from the first face 22 of the light guide plate 21 differ are input to the first diffraction grating member 30, and each of the parallel beams is diffracted and reflected as is while keeping a parallel beam state. Subsequently, the parallel beams diffracted and reflected advance while repeating total reflection between the first face 22 and second face 23 of the light guide plate 21, and input to the second diffraction grating member 40. The parallel beams input to the second diffraction grating member 40 are excluded from total reflection conditions by having been diffracted and reflected, and are emitted from the light guide plate 21, and are guided to the pupil 50 of the observer. Diffraction and reflection are performed multiple times at the second diffraction grating member 40, but the amount of light emitted from the light guide plate 21 based on each of the diffraction and reflection is reduced as the number of times of diffraction and reflection increases.
There is a symmetric relation between an interference pattern shape formed within the second diffraction grating member 40, and an interference pattern shape formed within the first diffraction grating member 30 as to a virtual plane perpendicular to the axis line of the light guide plate 21. Accordingly, the parallel beams to be diffracted and reflected at the second diffraction grating member 40 are diffracted and reflected with the same angle as the angle input to the first diffraction grating member 30, so a display image is not blurred, and is displayed at the pupil 50 with high resolution. Note that the first diffraction grating member 30 has the same specifications (e.g., thickness and maximum diffraction efficiency) as those of the second diffraction grating member 40.